The present invention relates to a water-turbine runner having a welded structure using a high strength and tough steel plate, and to a process for manufacturing the water-turbine runner.
In order to improve power generating efficiency in recent years, hydro-electric power generation has tended to have a higher and higher lift or head. This tendency requires a water-turbine runner of accordingly higher strength and toughness.
In the prior art, the water-turbine runner has been made of 13Cr steel, i.e., martensite stainless steel which is excellent in cavitation resisting characteristics. This 13Cr steel does not contain Ni. In accordance with the requirement for the output rise and the efficiency improvement of a single water-turbine runner, however, there has been developed 13Cr steel which contains Ni and has an excellent toughness. There has been proposed a water-turbine runner which is one piece cast or plural pieces separately cast of the 13Cr steel as is disclosed in Japanese Patent Publication No. 60-53737.
However, the prior art described above has the following problems:
a) A sufficient toughness cannot be achieved because of defects intrinsic to the casting such as a shrinkage cavity and segregation. PA1 b) For detecting the defects, a non-destructive inspection is used such as the ultrasonic flaw detection or the radiation inspection, but this requires a number or steps for inspecting the whole structure of the three-dimensional water-turbine runner with high precision. PA1 c) The casting defects detected by the non-destructive inspection are remedied by arc welding with covered electrodes, in which the 13Cr steel is liable to incur a weld crack. In order to avoid this weld crack, pre-and post-heating steps are required which undesirably increases the number of steps in the remedial operations. PA1 d) The finishing of the cast surface of the water turbine runner requires a great number of steps.